Transition test suite to use hashed storage

Hashed storage was recently added to GitLab and may have unknown bugs still lurking. Per https://gitlab.com/gitlab-com/infrastructure/issues/2821#note_49656410 , it would be nice to transition spec/ (and possibly features/) to enabled hashed storage for repositories by default. This will generate some false positives, but may also allow us to find genuine bugs and prevent regressions.